Lyon Mandelcorn

Tracking and ARC resistance of materials

Arc resistance by the common usage definition refers to the ability of an insulator to sustain surface discharges, at currents of 10 ma and higher, without electrical or structural damage. Here moisture usually is not a significant factor among the effects, or causes, of these discharges, or arcs. In the case of tracking, on the other hand, the damaging discharges are usually less than 10 ma, and are initiated and sustained by the presence of moisture and dust. Moisture also causes tracking failures along the interior of certain insulators, as well as surface tracks. The life-time of insulation that has both low track and arc resistance, e.g., phenolic resin, can be of the order of months under tracking conditions or may take less than one second of arcing before failure occurs. In contrast to corona effects on insulation, which are due to chemical reaction of the active products of the corona discharges at currents less than 1 ma, the effects of both arcing and tracking discharges are essentially thermal, i.e., they involve high-temperature reactions such as decomposition or combustion. (1,2)

High voltage power capacitor dielectrics recent developments

This paper discusses recent, new impregnant developments and evaluations for capacitor dielectrics. These dielectric fluids are isopropylbiphenyl (Wemcol), di-2 ethylhexyl phthalate plus trichlorobenzene (Dielektrol II) butylated monochlorodiphenyl oxide (Edisol) phenyl xylyl ethane (PXE) and benzyl neocaprate (BMC). The film-paper dielectrics impregnated with these were extensively tested for thermal stability under voltage and resistance to overvoltages with partial discharges: Considerable attention is now being devoted to practically significant partial discharge testing, to determine service reliability. Also included here is the evaluation of windings where one foil is narrower and folded at the edges to reduce the voltage stress at the foil edges.

Inclusion compounds as purification tools.

An inclusion compound consists primarily of two molecular species, one of which provides space in its structure in which molecules of the other are accommodated. These two species are known as the “host” which forms the essential structure, and the “guest” which is included, or contained, in this structure. A specific host will form an inclusion compound with different molecules provided their dimensions satisfy the geometrical restrictions imposed by the available spaces in the structure. The interaction between host and guest is generally quite weak, the bonds being of the van der Waals type, and each of these two constituents retains many of the properties it has when separate. An inclusion compound cannot be formed with molecules as guests whose own free energy of interaction in the condensed phase is more negative than that which they might have with the host structure. Most inclusion compounds are solids. Some inclusion compounds are found in the liquid state. There are three categories into which inclusion compounds are classified, and these are based on the geometrical form of the space in which the guest molecules are located.‘ The clathrate or cage structure is one in which one or more guest molecules are completely enclosed by host molecules. Decomposition of a clathrate can occur only if the host structure is broken by such means as melting, sublimation or dissolution. There are channel structures, and these are composed of long channels that are open at the ends and contain the guest molecules. Only the cross-sectional dimension of a molecule is critical for it to be a guest, and this is set by the smallest constriction that occurs in the channel space. Release of guest molecules, from an open end of a channel, is possible without first breaking the host structure. In the third category of inclusion compounds in this listing are those of the layer type. Here, the molecules or atoms of the host are arranged in layers between which guest molecules may be accommodated. Layer structures are fairly flexible and there is generally a wide variety of different molecules of various sizes and shapes that can be accommodated in a given layer structure. In fact, the included molecule parttally determines the interlayer distance. This effect should be compared with the fact that the clathrate and channel structures are quite inflexible, and are, therefore, much more selective in regard to size and shape of guest molecules than are the layer host compounds. The host structure of some inclusion compounds exists only with an appropriate guest component. Otherwise, the unassociated solid host molecules

The effects of electrical discharges on solid insulation

The behavior of an insulator when subjected to electrical discharges often determines whether it can be used in a particular electrical device. There are various tests available for evaluating the discharge resistance (arc resistance) of insulation but their results do not correlate very well with field behavior. A more reliable testing procedure is needed which also yields information on the breakdown mechanism occurring in insulation subjected to various conditions of discharge.